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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Frame check sequence</span></span>
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<p>A <b>frame check sequence</b> (<b>FCS</b>) is an <a href="Error-detecting_code" class="mw-redirect" title="Error-detecting code">error-detecting code</a> added to a <a href="Frame_(networking)" title="Frame (networking)">frame</a> in a <a href="Communication_protocol" title="Communication protocol">communication protocol</a>. Frames are used to send <a href="Payload_data" class="mw-redirect" title="Payload data">payload data</a> from a source to a destination.
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<div class="mw-heading mw-heading2"><h2 id="Purpose">Purpose</h2></div>
<p>All frames and the bits, bytes, and fields contained within them, are susceptible to errors from a variety of sources. The FCS field contains a number that is calculated by the source node based on the data in the frame. This number is added to the end of a frame that is sent. When the destination node receives the frame the FCS number is recalculated and compared with the FCS number included in the frame. If the two numbers are different, an error is assumed and the frame is discarded.
</p><p>The FCS provides error detection only. Error recovery must be performed through separate means. <a href="Ethernet" title="Ethernet">Ethernet</a>, for example, specifies that a damaged frame should be discarded and does not specify any action to cause the frame to be retransmitted. Other protocols, notably the <a href="Transmission_Control_Protocol" title="Transmission Control Protocol">Transmission Control Protocol</a> (TCP), can notice the data loss and initiate retransmission and error recovery.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup>
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<div class="mw-heading mw-heading2"><h2 id="Implementation">Implementation</h2></div>
<p>The FCS is often transmitted in such a way that the receiver can compute a running sum over the entire frame, together with the trailing FCS, expecting to see a fixed result (such as zero) when it is correct. For <a href="Ethernet" title="Ethernet">Ethernet</a> and other <a href="IEEE_802" title="IEEE 802">IEEE 802</a> protocols, the standard states that data is sent least significant bit first, while the FCS is sent most significant bit (bit 31) first. An alternative approach is to generate the bit reversal of the FCS so that the reversed FCS can be also sent least significant bit (bit 0) first. Refer to <a href="Ethernet_frame#Frame_check_sequence" title="Ethernet frame">Ethernet frame ยง Frame check sequence</a> for more information.
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<div class="mw-heading mw-heading2"><h2 id="Types">Types</h2></div>
<p>By far the most popular FCS algorithm is a <a href="Cyclic_redundancy_check" title="Cyclic redundancy check">cyclic redundancy check</a> (CRC), used in Ethernet and other IEEE 802 protocols with 32 bits, in <a href="X.25" title="X.25">X.25</a> with 16 or 32 bits, in <a href="HDLC" class="mw-redirect" title="HDLC">HDLC</a> with 16 or 32 bits, in <a href="Frame_Relay" title="Frame Relay">Frame Relay</a> with 16 bits,<sup id="cite_ref-cisco-technote-fr_3-0" class="reference"><a href="#cite_note-cisco-technote-fr-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> in <a href="Point-to-Point_Protocol" title="Point-to-Point Protocol">Point-to-Point Protocol</a> (PPP) with 16 or 32 bits, and in other <a href="Data_link_layer" title="Data link layer">data link layer</a> <a href="Communications_protocol" class="mw-redirect" title="Communications protocol">protocols</a>.
</p><p>Protocols of the <a href="Internet_protocol_suite" title="Internet protocol suite">Internet protocol suite</a> tend to use <a href="Checksum" title="Checksum">checksums</a>.<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup>
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<div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2></div>
<ul><li><a href="Syncword" title="Syncword">Syncword</a></li></ul>
<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>
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</style><cite class="citation cs2">"3.1.1 Packet format", <a rel="nofollow" class="external text" href="https://ecfsapi.fcc.gov/file/1050839507018/IEEE%20Standard%20for%20Ethernet.pdf"><i>802.3-2012 - IEEE Standard for Ethernet</i></a> <span class="cs1-format">(PDF)</span>, <a href="IEEE_Standards_Association" title="IEEE Standards Association">IEEE Standards Association</a>, 2012-12-28<span class="reference-accessdate">, retrieved <span class="nowrap">2015-07-05</span></span></cite></span>
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<li id="cite_note-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-4">^</a></b></span> <span class="reference-text"><cite class="citation cs1"><a rel="nofollow" class="external text" href="https://datatracker.ietf.org/doc/html/rfc1071"><i>Computing the Internet Checksum</i></a>. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://doi.org/10.17487%2FRFC1071">10.17487/RFC1071</a></span>. <a href="Request_for_Comments" title="Request for Comments">RFC</a> <a rel="nofollow" class="external text" href="https://datatracker.ietf.org/doc/html/rfc1071">1071</a>.</cite></span>
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